#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
using namespace std;
int main()
{
	int i, N, sum = 0;
	cin >> N;
	for (i = 0; i < N; i++)
	{
		if (i % 2 == 1)
		{
			sum = sum + i;
		}
	}
	cout << sum << endl;
	return 0;
}